1. Field
This application relates to hollow fiber membrane fluid transport devices, specifically to the method of manufacturing such membrane fluid transport devices, and even more specifically to the means of assembling the hollow fibers into bundles and sealing the ends of the hollow fibers to make suitable contactors.
2. Background of the Invention
Membrane contactors are useful devices for separation processes, contacting processes, or as filters. A membrane contactor includes a membrane or membranes held in such a manner as to separate two regions of flow and enable the membrane to act as a separation means between the two phases, and a housing to enclose the membrane and contain and direct the flow of the multiple phases. The membrane acts as a barrier between the two fluid phases and selectively allows or prohibits the transport of one or more chemical species or particles from one fluid stream to the other. The housing has one or more ports to allow flow to and from the membrane. Membrane contactors can be considered as a subclass of the more general class of fluid or fluid/gas transport devices.
Membrane contactors have applications as filters, separation systems, or contacting devices in many industries such as chemical, pharmaceutical, food and beverage, environmental, water treatment, and semiconductor processing. Membrane separation processes such as gas/liquid separation or membrane distillation are replacing their bulk counterparts (distillation towers, stripping columns) due to improved energy efficiency, scalability, the ability to operate isothermally, and smaller physical footprints. In addition, membrane filters, separators, and contactors generally have no moving parts and are physically simple and rugged, resulting in low maintenance cost.
Hollow fiber membrane devices are one class of membrane modules that employ membranes in hollow fiber form. While many types of membranes are available in sheet form, the ability to create significantly higher surface area per unit volume with a hollow fiber membrane is of major advantage to the designer and user of a membrane filter or contactor. A hollow fiber membrane is also typically self-supporting in contrast to flat sheet or thin film membranes that usually require a skeletal structure for support. In addition, typical contactor designs employing hollow fiber membranes, whether constructed as a cross flow element or in a dead-end configuration, offer more uniform flow and fewer regions for the flow to stagnate.
The usefulness and efficiency of a membrane contactor is determined by the available surface area of the membrane per unit volume of the device and the rate at which the transfer or removal of the species of interest occurs; this is generally governed by the flux (flow per unit area, per unit time, per unit pressure gradient) of the process stream. The available surface area for a hollow fiber membrane module is dictated by the packing density of the fibers (the ratio of the sum of the cross sections of the individual fibers to the total available cross sectional area). The higher the packing density and the greater the surface area to volume ratio generally results in a more efficient module.
Two other useful parameters for defining the performance of a porous membrane are the pore size distribution and the porosity. The pore size distribution is a statistical distribution of the range of pore diameters found in the membrane wall. The largest pore size is also generally characterized by a measurement called a bubble point, which is defined in the below detailed description of the invention. The smaller the mean pore size, the smaller the particle a membrane filter will separate.
The porosity of a hollow fiber membrane may be defined as the percentage of free volume in the membrane, or, for PTFE hollow fiber membranes, as (1−(membrane density/2.15)*100 where 2.15 is the density of solid PTFE. The higher the porosity, the more free volume and generally the higher the flux rate through the membrane wall.
For a given pore size distribution, higher porosities are often desirable as they lead to higher flux rates. Unfortunately higher porosities also generally lead to softer membrane walls, causing the hollow fibers to be structurally very soft and prone to deformation and collapse, especially during a potting process. Heating the ends of the hollow fibers reduces the porosity and hardens the heated portion of the fibers, reducing the likelihood of the fibers being crushed or deformed on compression.
The elements of a hollow fiber membrane contactor include the hollow fiber membrane itself, the housing, and a means to bind the fibers to one another and to the housing. A hollow fiber membrane is a porous or non-porous, semi-permeable membrane of defined inner diameter, defined outer diameter, length and pore size, and generally of a very high aspect ratio, defined as the ratio of the length to the diameter of the fiber. A hollow fiber membrane contactor is generally comprised of a plurality of fibers.
The housing is an outer shell surrounding the membrane that secures and contains a potted bundle of hollow fibers. The housing is equipped with one or more inlets and one or more outlets, such that the potted bundle of hollow fiber membrane acts as a barrier and separates the two phases or process streams. The design of the housing, and specifically the relationship of the inlets and outlets, regulates the flow of the process fluid into or out of the fiber lumens and directs the processed fluid away from the device. There are typically two common modes of designing the housing, which relate to how the fluids interact with the membrane. What are known to those well versed in the art as dead-end elements consist of a housing that directs all of the volume of one fluid to pass through the membrane walls to reach the discharge or exit of the housing. The dead-end design is a very common design employed for membrane filtration. For dead-end hollow fiber membrane filters, both ends of each hollow fiber membrane are potted or bound at one end of the housing. In dead-end hollow fiber membrane filters the process fluid either enters the lumens of the hollow fibers and discharges out through the walls of the hollow fiber membrane, or enters through the walls and discharges out of the lumens. In either case, this ensures that the entire process stream passes through the membrane wall.
A dead-end hollow fiber membrane filter configuration is contrasted to a cross flow configuration in which the lumens are open at both ends, and only a portion of the process stream entering the upstream lumens passes through the membrane wall, while the remainder of the fluid discharges through the downstream lumen openings. The portion of the fluid discharging from the downstream lumen end may be passed along to another membrane element, recycled to the beginning of the unit, or discarded. The cross flow configuration mode is employed with both filtration as well as membrane contacting or separation processes.
A hollow fiber membrane bundle may be integral to the housing or may be designed so that the potted hollow fiber membrane bundle may be installed and removed.
To create a membrane filter or membrane separator or contactor module, one must establish a suitable means for binding the hollow fiber membranes into an integral bundle and sealing the exposed ends of the hollow fibers from the body of the module, a process hereafter referred to as potting the fibers. Potting the hollow fiber membranes may occur prior to, or during the operation of mounting the hollow fiber membranes into the housing. To bind the ends of the hollow fibers to one another, a potting compound is employed. A potting compound is a material that when applied around the ends of hollow fibers, bonds them together into a solid, cohesive mass that isolates and fixes the hollow fibers from the remainder of the bundled assembly of fibers.
A potted bundle of hollow fibers is a plurality of hollow fiber membranes bound together or potted at least at one end. Both ends may be potted, or the ends of each individual fiber may be looped back in a U-shape and potted at or near one end. One potential configuration can be where the bundled fibers are first twisted 180 degrees and then folded into itself to form a closed end and an open end with the open end potted, i.e.—embedded in a solid mass providing a fluid-tight seal around each fiber. There may be several themes and variations on these basic configurations.
Membranes for contactors or filters have been developed from a variety of synthetic polymers and ceramics and have been known in the industry for many years. While ceramic membranes offer the chemical resistance and high service temperature required by aggressive acidic, alkali, or organic solvent applications, in their present-day state they are very fragile, very expensive, and very difficult to work with, a combination of features that keeps ceramic membranes out of many applications.
The vast majority of state of the art polymeric membranes are limited as they are not inert, they possess inadequate chemical purity, thermal stability and chemical resistance, and occasionally have undesirable surface properties, preventing their use in certain important applications. This is because these very same membranes are spun from solution, and the fact that they must be soluble in certain solvents to convert to a membrane means that the final membrane itself is susceptible to attack by those same classes of solvents.
It has long been desired to be able to have membranes manufactured from fluorinated or perfluorinated resins due to their high service temperatures, chemical stability, inertness, and chemical resistance to a wide range of solvents, acids and alkali systems. However, membranes produced from non-fully fluorinated polymers still require aggressive solvent systems and very high processing temperatures to manufacture, increasing cost and generating environmental and waste issues. Membranes manufactured from Polytetrafluoroethylene (hereafter referred to as PTFE) are most desirable because, as a fully fluorinated polymer, they offer the best combination of thermal and chemical stability of all the fluorinated and perfluorinated resins commercially available. In addition, the method by which they are converted to membranes does not employ hazardous solvent systems; instead using a stretching and orientation method.
It is also desirable to have membranes manufactured from fluorinated or perfluorinated resins, especially fully fluorinated resins, due to their low surface energy Filtration of organic liquids, separating organic from aqueous systems, or removing vapor from aqueous systems all favor low energy membranes. PTFE offers the lowest surface energy of all the fluorinated or perfluorinated polymeric membranes—less than about 20 dyne-cm.
Current potting materials have many limitations such as inadequate chemical resistance, lack of chemical purity and inertness, and poor thermal stability. They are also very difficult to use, and produce inefficient and costly modules. One such class of inadequate potting systems consists of low viscosity materials including urethanes and epoxies which are easy to apply but are chemically very impure and are not chemically resistant, nor do they offer high service temperatures.
It is also therefore highly desirable to have a potting compound that has excellent chemical resistance and high service temperatures that would match those of the fluorinated, perfluorinated, or fully fluorinated membrane, because the effectiveness of a contactor constructed with a fluorinated or perfluorinated membrane for a thermally or chemically aggressive system is limited by the weakest part of the device. An effective combination of a potting system for a fluorinated or perfluorinated membrane has hitherto been unavailable. Current potting methods are not amenable to the use of fluorinated or perfluorinated compounds; they cannot produce membrane modules with high fiber packing density, or with economical manufacturing cycle times; nor can they be employed to make contactors with relatively soft fibers or contactors containing many thousands of fibers, something necessary for many commercial membrane applications.